Cumene is widely used in many chemical industries for the manufacture phenol, acetone, α-methyl styrene, etc.
In the prior art, various processes are demonstrated for the preparation of cumene. Normally alkylation of benzene to cumene is done by Friedel-Crafts alkylation route. The Friedel-Crafts alkylation reaction is catalyzed by both protons (e g. H3PO2) and Lewis acids (e.g. BF3) on different supports, such as amorphous and crystalline aluminosillicates. The reaction proceeds through activation of the olefin which then reacts with benzene. Riedel type of mechanism proposed interaction between free benzene molecule and activated olefin molecule attached to an active site (“Friedel Craft Alkylation Chemistry”, Robert R. and Khalaf, A., Marcel Dekker Inc. New York, 1984).
U.S. Pat. No. 4,169,111, 1997 teaches isopropylation of benzene in a liquid phase catalytic reaction and is operated at very close to critical conditions of the reactants isoproplyl alcohol (IPA) and benzene. Harper et al, ‘Industrial and Laboratory Alkylations’ (edited by L. F. Albrith and A. R. Goldsby, ACS Symposium Series, 55, 371, ACS, Washington D.C., 1977) studied the kinetics of the reaction. The equilibrium constant for the transalkylation reaction decreases with increasing temperatures. This means that the cumene content at equilibrium is favored by decreasing the temperature, although the effect is not particularly good.
The two processes most widely used on industrial scale are UOP's Cumox process (U.S. Pat. No. 4,128,593, 1978) and Monsanto-Lummus Cumene process (T. Vett, ‘Monsanto Lummus styrene process is efficient’ Oil & Gas Journal, 76 July 1981). The Cumox process for the production of phenol is accomplished via UOP's catalytic condensation process for cumene. The process utilizes a solid phosphoric acid (SPA) catalyst. In this process a mixture of propylene with excess benzene is pumped upwards in the alkylation reactor filled with SPA catalyst. The reactor effluent is routed through a two stage flash system prior to the final fractionator. Excess of the benzene gets separated in the flash system and is recycled back to the alkylation reactor. Cumene as a major product is separated in the fractionation column. The bottom from the fractionator comprises highly aromatic material containing mainly diisopropylbenzene (DIPB) isomers. The DIPB is reacted back with, benzene in a mole ratio of 1:8 to obtain cumene in a transalkylation reactor. A very small and regulated amount of water is added to the feed in order to keep the catalyst away from becoming friable and disintegratable. The process offers 99.3% by wt. conversion of propylene with 92.5% selectivity to cumene.
In Monsanto-Lummus Crest process, dry benzene and propylene are mixed in the alkylation reactor with a aluminium chloride hydrogen—chloride catalyst at controlled temperature, catalyst concentration and residence time. The reactor effluent is washed with water and caustic to separate the organics from strongly acidic catalyst. Major features of this process are low benzene recycle ratio.
A process using zeolite catalyst is also demonstrated (U.S. Pat. No. 5,687,540). According to this process, a heterogeneous catalyst (large pore 12 membered ring zeolite-β) based process was used for the production of cumene by isopropylation of benzene using isopropyl alcohol. The process uses a mole ratio of IPA: benzene of 1:6, catalyst volume of 4 cc per 100 cc of reactor volume and space velocity of 2.5 h−1. The two processes described above (UOP and Monsanto) use isopropylene (which is usually obtained from isopropanol) as the reagent while the process, of this patent uses isopropanol directly, thereby reducing one step of isopropanol dehydration to isopropylene. In addition isopropanol is more stable than isopropylene at reaction condition. The use of isopropanol prolongs the life of the catalyst.
The processes documented in the literature and as discussed above however, have several drawbacks. The Cumos process uses excess of benzene, which needs to be separated by flash condensation and then needs to be recycled. The flash condensation involves additional operating cost. Also a higher reactant mole ratio is required. The formed cumene needs to be separated by using large distillation columns. Isomers of DIOB are formed as byproducts, which need to be transalkylated to cumene. This involves additional transalkylation reactor. The catalyst used in the process is corrosive and creates environmental pollution. In addition water needs to be added to keep the catalyst away from becoming friable and disintegrating. The Monsanto process uses strong acid catalyst, which is corrosive in nature. The reactor effluent in this process needs to be washed with water and caustic in order to separate organics. The process using zeolite catalyst also suffers from certain drawbacks like requirement of higher catalyst volume, higher reactant mole ratio and byproduct formation (e.g. DIPB isomers, toluene, C8, C10 and C11 aromatics, n-propyl benzene, high boiling fractions). The processes documented in the literature thus have several drawbacks like high catalyst volume, high feed mole ratio, lower space velocity, lower yields, byproduct formation, higher capital and operation cost, corrosion problem etc. In order to make the process more economical and eco-friendly, attention needs to be given to eliminate above drawbacks.